The present invention relates to a solid-state image sensor and a method of driving the same, and more particularly to an amplifying type solid-state imaging device such as a MOS image sensor having an amplifying function per unit pixel, and also to a method of driving such an image sensor.
FIG. 14 shows an exemplary structure of a conventional two-dimensional solid-state image sensor known heretofore as an amplifying type solid-state imaging device in the related art. In this diagram, a unit pixel 105 is composed of a photodiode 101, an amplifying MOS transistor 102, a photodiode reset MOS transistor 103, and a vertical select MOS transistor 104. In this structure, a gate electrode of the photodiode reset MOS transistor 103 is connected to a vertical reset line 108, a gate electrode of the vertical select MOS transistor 104 to a vertical select line 109, and a source electrode of the vertical select MOS transistor 104 to a vertical signal line 110, respectively.
A horizontal select MOS transistor 112 is connected between one end of the vertical signal line 110 and a horizontal signal line 111. The operation of each pixel is controlled per row by two kinds of vertical scanning pulses xc3x8VSn and xc3x8VRn outputted from a row-select vertical scanning circuit 113, and a pixel signal is outputted to the horizontal signal line 111 via the horizontal select MOS transistor 112 which is controlled by a horizontal scanning pulse xc3x8Hm outputted from a column-select horizontal scanning circuit 114. At this time, the signal charge stored in the photodiode 101 through photoelectric conversion is converted into a signal current by the amplifying MOS transistor 102 and then is delivered as an output signal of the image sensor.
However, in the known amplifying type two-dimensional solid-state image sensor of the above structure, there exists a problem of characteristic deviation in the active elements constituting each pixel, principally in the amplifying MOS transistor 102, and particularly relative to deviation of the threshold voltage Vth of the MOS transistor. And such deviation is included directly in the output signal of the image sensor. Since this characteristic deviation has a fixed value per pixel, it appears as a fixed pattern noise (FPN) in the picture displayed on a screen. For suppressing such fixed pattern noise, it is necessary to externally connect to the device a noise elimination circuit using a frame memory or a line memory, so as to eliminate any noise component derived from the characteristic deviation in the pixel. As a result, in a camera system or the like employing such a solid-state image sensor as an imaging device, the scale thereof is rendered larger correspondingly to the noise elimination circuit connected thereto externally.
In comparison with the above, there is contrived another amplifying type solid-state image sensor which has a structure of FIG. 15 and is capable of internally suppressing such fixed pattern noise in the device. The difference of this solid-state image sensor resides in the point that, although its unit pixel 105 is structurally the same as FIG. 14, a horizontal output circuit 115 is provided for suppressing the fixed pattern noise derived from the characteristic deviation in the pixel 105, and this horizontal output circuit 115 executes a process of taking the difference between pre-read and post-read (pre-reset and post-reset) signals of the pixel 105.
In FIG. 15, a load MOS transistor 116 serving as a load to the source follower operation of an amplifying MOS transistor 102 is connected between a vertical signal line 110 and the ground. Further, one main electrode of each of paired signal switch MOS transistors 117 and 117xe2x80x2 is connected to the vertical signal line 110. And a pair of signal holding capacitors 118 and 118xe2x80x2 are connected respectively between the ground and the other main electrodes of such paired signal switch MOS transistors 117 and 117xe2x80x2.
Further a pair of horizontal select MOS transistors 112 and 112xe2x80x2 are connected respectively between the other main electrodes of the paired signal switch MOS transistors 117, 117xe2x80x2 and a pair of horizontal signal lines 111, 111xe2x80x2. And a noninverting (+) input terminal and an inverting (xe2x88x92) input terminal of a differential amplifier 119 are connected respectively to the pair of horizontal signal lines 111 and 111xe2x80x2.
In the amplifying type solid-state image sensor of the above structure, pixel pre-reset and post-reset signals are held respectively in signal holding capacitors 118, 118xe2x80x2 via the signal switch MOS transistors 117, 117xe2x80x2 and then are supplied to the differential amplifier 119 via the horizontal select MOS transistors 112, 112xe2x80x2 and the horizontal signal lines 111, 111xe2x80x2. Subsequently, the difference between the pixel pre-reset and post-reset signals is taken in the differential amplifier 119 to thereby eliminate the fixed pattern noise derived from the characteristic deviation in each unit pixel.
Although it is possible in the amplifying type solid-state image sensor of the above structure to suppress the fixed pattern noise derived from the characteristic deviation in each unit pixel, the pixel pre-reset and post-reset signals reach the differential amplifier 119 via separate signal paths, so that the characteristic deviations relative to the paired signal switch MOS transistors 117, 117xe2x80x2 and the paired horizontal select MOS transistors 112, 112xe2x80x2 appear in the picture as fixed pattern noises with vertically correlated streaks. Therefore, this structure also requires an external correction circuit for suppressing the fixed pattern noises with vertical streaks.
It is an object of the present invention to provide an improved amplifying type solid-state image sensor which is capable of suppressing, within the device, any fixed pattern noise derived from characteristic deviation in each unit pixel and also other fixed pattern noise of vertical streaks. And another object of the invention is to provide a method of driving such an image sensor.
According to a first aspect of the invention, there is provided a solid-state image sensor which comprises unit pixels each having a photoelectric conversion element for converting incident light into electric signal charge and then storing the signal charge obtained through such photoelectric conversion, an amplifying element for converting into an electric signal the signal charge stored in the photoelectric conversion element, and a select switch for selectively outputting the pixel signal from the amplifying element to a signal line; and a reset circuit in each of the unit pixels for resetting the photoelectric conversion element every time a pixel signal is outputted from the relevant unit pixel.
According to a second aspect of the present invention, there is provided a method of driving a solid-state image sensor of the above structure. The method comprises the steps of resetting the photoelectric conversion element every time a pixel signal is outputted from the relevant unit pixel; then delivering a pre-reset signal and a post-reset signal from each unit pixel and taking the difference between the pre-reset signal and the post-reset signal.
And according to a third aspect of the present invention, there is provided a camera which comprises an optical system for focusing incident light from an object scene to form an image thereof; a solid-state image sensor comprising unit pixels each having a photoelectric conversion element for converting the optical image formed by the optical system into electric signal charge and then storing the signal charge obtained through such photoelectric conversion, an amplifying element for converting into an electric signal the signal charge stored in the photoelectric conversion element, and a select switch for selectively outputting the pixel signal from the amplifying element to a signal line, and a reset circuit in each of the unit pixels for resetting the photoelectric conversion element every time a pixel signal is outputted from the relevant unit pixel; a drive capable of driving the solid-state image sensor; and a signal processor for processing the output signal of the solid-state image sensor.
In each of the unit pixels constituting the solid-state image sensor of the structure described above, the photoelectric conversion element is reset every time a pixel signal is outputted, so that a pre-reset signal and a post-reset signal per pixel are outputted successively from each of the unit pixels. In this case, fixed pattern noise derived from any characteristic deviation in the pixel is generated as an offset component from the amplifying element of each pixel. Therefore, the noise component can be canceled by taking the difference between the pre-reset signal and the post-reset signal. Further in the two-dimensional solid-state image sensor, the pre-reset and post-reset signals are outputted from a vertical signal line to a horizontal signal line via a common signal path, so that fundamentally none of vertically correlated streak noise components is generated.